I am studying the influences of gonadal steroid hormones on the anatomical and physiological properties of action potential- generating (i.e. electrically excitable) cells. I am using the electromotor system of mormyrids, a group of weakly electric fish from Africa, as a model system. Mormyrids have an electric organ located in the tail that consists of modified muscle cells called electrocytes that together generate an Electric Organ Discharge (EOD). The characteristic properties of the EOD are determined by the anatomy and physiology of the electrocytes. The activity generated by a single electrocyte determines the appearance of the entire EOD. I have found that gonadal steroid hormones (e.g. testosterone) can induce changes in the EOD that mimic natural sex differences. Steroid-induced changes in the EOD are correlated with changes in the morphology of the electrocyte's excitable membranes and the duration of their action potential waveforms. I want to continue studying the cellular mechanisms underlying hormone- induced changes in the anatomy and physiology of the electric organ. I also want to extend my anatomical studies of steroid- binding cells in the brain of electric fish. The effects of steroid hormones on electrocytes may be fundamental to their action on other classes of electrically excitable cells, namely neurons and muscle fibers. The EOD itself is somewhat unique in that it is both a behavior, important in an electrical guidance system and in social communication, and a discrete neurophysiological event. In this way, we can weave an interface between steroid hormone action at functional (i.e. behavior) and mechanistic or cellular levels.